5 gallon or 10 gallon round cooler?

I'm trying to decide between a 5 and 10 gallon MLT. Should I really be concerned about dead space in the MLT?

No, don't worry about deadspace. Get the 10 gallon. If you want to do all-grain, you will need to have one bigger than 5 gallons.

Don't forget that you will have a false bottom, plus you are going to have a bunch of grains, plus water in there.

Go for the 10- it will give you the flexibility to both BIG 5 gallon batches and moderate sized 10 gallon batches. With a 5 gallon cooler, your max grain bill will be about 15# if you do a thick (1 qt/lb) mash. If you are concerned about dead space and temperature loss, a two-inch thick piece of foam insulation laid on top of the grain bed will give you rock-solid temperature stability, IME.

Something like this:

When brewing big beers, it's really nice to have the extra space for grain.

Surely go for the 10 gallon, you won't regret it, I think the home depot 10 gallon is 39.99, well that's what I paid anyways last winter.

Go for the 10 Gallon Cooler. I started with a five and quickly ended up upgrading to the ten. More room for large grain bills, and mash out if you want to do one.

10 gallon definitely. see my thread titled "parti-gyle file"